A number of conventional encoding methods of audio signals such as voice and music are known. As one such example, a so-called transform coding method which converts a time-domain audio signal into a frequency-domain spectral signal (spectral transformation) can be cited.
As the above-mentioned spectral transformation, for example, there is a method of converting the audio signal of the time domain into the spectral signal of the frequency domain by blocking the inputted audio signal for each preset unit time (frame) and carrying out Discrete Fourier Transformation (DFT), Discrete Cosine Transformation (DCT) or Modified DCT (MDCT) for each block.
Further, when encoding the spectral signal generated by the spectral transformation, there is a method of dividing the spectral signal into frequency domains of a preset width and quantizing and coding after normalizing for each frequency band. A width of each frequency band when performing frequency band division may be determined by taking human auditory properties into consideration. Specifically, there is a case of dividing the spectral signal into a plurality of (for example, 24 or 32) frequency bands by a band division width called the critical band which grows wider as the band becomes higher. Furthermore, encoding may be carried out by conducting adaptable bit allocation per frequency band. For a bit allocation technique, there may be cited the technique listed in “IEEE Transactions of Acoustics, Speech, and Signal Processing, Vol. ASSP-25, No. 4, August 1977” (hereinafter referred to as Document 1).
In the Document 1, bit allocation is conducted in terms of the size of each frequency component per frequency band. In this technique, a quantization noise spectrum becomes flat and noise energy becomes minimum. However, since a masking effect and an isosensitivity curve are not taken into consideration aurally, an actual noise level is not minimum.
Further, in the Document 1, a concept of the critical band is utilized and quantization is made collectively by the higher-the-wider band division width, and hence, as compared to the low band, there is a problem of deteriorating information efficiency in securing quantization accuracy. Moreover, to solve this problem, there is a need of an additional function such as a method of separating and extracting only a specified frequency component from one frequency band and a method of separating and extracting a large frequency component in a preset time domain.